Rendering faster the transparent areas in Cycles
Branched path tracing is significantly faster on empty background areas since it limits initial samples and instead gathers multiple subsamples: What is branched path tracing and how is it useful?

Some effects such as depth of field or motion blur may need a lot of initial samples anyway though, so branched path may not always be faster. Additionally, it consumes quite a lot of extra memory when running on GPU, so it can easily put a scene over the edge of fitting in VRAM or not.